Various processes for extracting hydrocarbons from oil shale have long been known. They generally involve retorting the shale, that is, heating it to a high enough temperature so that the hydrocarbon material contained in the rock is pyrolyzed or broken down. Once this is accomplished, the hydrocarbons generally separate from the rock substrate and are recovered in gaseous and liquid form. One commonly used method for recovery of the gaseous fraction is by sweep or carrier gas, wherein a relatively inert, noncondensable gas is passed through the retorting shale, carrying with it the gaseous hydrocarbons. Thereafter the hydrocarbons are condensed and removed from the carrier system. The most commonly used inert gas has been nitrogen due to its availability and low cost. Steam has also been used to an extent, however, it does produce some side reaction.
The present invention utilizes carbon dioxide as the carrier gas. Heretofore, carbon dioxide has not been used, presumably because of its higher cost relative to nitrogen. The use of carbon dioxide has however unexpectedly been found to be beneficial from the standpoint of oil yield, which can be substantially increased under controlled conditions in a carbon dioxide system.